A variety of retroreflective sheetings comprising microspheres have been used for traffic signs, road signs, pavement markings, conspicuity tapes, etc. Illustrative examples include those disclosed in U.S. Pat. No. 3,005,382 (Weber) and Japan-A-57-27748.
These sheetings typically comprise a polymeric binder layer and a plurality of microspheres, typically arranged in a monolayer, partially embedded in and protruding from the binder layer. The microspheres typically have a reflective layer thereon, e.g., aluminum, silver, or dielectric coating, or in other embodiments, the binder layer contains reflective particles, e.g., pigment particles such as titanium dioxide, metal flake, nacreous flakes, etc., which function in optical association with the microspheres to provide retroreflective effect.
These sheetings typically provide the most effective retroreflective performance at incidence angles that are substantially perpendicular to the plane of the monolayer of microspheres.
In many applications, such as traffic signs, a sign and the sheeting thereon are oriented substantially perpendicularly to prevalent the direction of approach and effective retroreflective effect is attained.
To improve retroreflective performance at high incidence angles, e.g., such as is encountered with sheetings used on the sides of vehicles, on the sides of tunnels, guard rails, road surfaces, etc., it has been known to form protrusions in the sheeting such that at least some of the microspheres are oriented so as to be presented more perpendicularly to the observer. This configuration has been used extensively in pavement marking tapes where effective retroreflection at very high incidence angles is desired. See, for example, U.S. Pat. No. 5,316,406 (Wyckoff).
Several approaches to achieving a support layer with protrusions bearing retroreflective elements are known.
In some embodiments, a foaming composition is applied to a desired spot on a flat substrate and a liquid composition, e.g., a paint, containing binder resins and reflective pigments is coated over the substrate surface. Microspheres are scattered on the surface of the liquid composition, becoming partially embedded therein. The composition is cured to secure the microspheres thereto and the foaming composition activated, e.g., with heat, to raise portions of the binder layer thereby forming protrusions with microspheres thereon. Examples of this approach are disclosed in Japanese Publication Nos. A-55-65524 and A-57-193352.
In some embodiments, a substrate made of thermoplastic resins is embossed to form a surface with protrusions thereon. A paint containing binder resins and reflective pigments is coated on the surface in sufficient thickness to fill the indented portions and yield a flat surface. Microspheres are partially embedded in the surface the binder resins cured to secure them there. The construction is then heated to cause the substrate to return to a flat shape, thereby deforming the binder layer to yield protrusions corresponding to the different in coated thickness of the binder layer. Examples of this approach are disclosed in Japanese Publication Nos. A-53-46363 and A-53-46371.
In some embodiments, a paint which shrinks and forms wrinkles is used. The paint, which contains binder material and reflective pigments, is coated on a substrate, microspheres are partially embedded therein, and the paint then heated to dry it and secure the microspheres therein, and to also cause it to wrinkle, thereby forming protrusions with microspheres thereon. An example of this approach is disclosed in Japanese Publication No. A-57-10102.
Other approaches include forming protrusions and partially embedding microspheres therein as disclosed in U.S. Pat. No. 4,069,281 (Eigenmann) and Japanese Publication No. A-58-237243. GB Patent No. 2,251,091 discloses a retroreflective sheeting comprising microspheres adhered to an aluminum layer and German Patent No. 3,039,037 discloses a vehicle tire with a side face having raised areas to which microspheres are adhered.
Each of these approaches suffers some deficiency. It is frequently difficult to control the depth to which the microspheres are embedded because the microspheres are typically applied to the binder layer before it is cured and/or dried and thus it is still quite soft. It is also difficult to form protrusions of desired shape and size in which the microspheres are uniformly embedded. If the microspheres are not embedded deeply enough, they may tend to be easily dislodged. If the microspheres are too deeply embedded the retroreflective response may be impaired. If the microspheres are not uniformly embedded, the product may not provide a desired uniformity of retroreflective effect. Finally, it is also often difficult in these approaches to form protrusions in the shape of pyramids and prisms which would be advantageous for retroreflection at very high incidence angles.